The hormonal regulation of maternal responsiveness in primates has been poorly investigated. Preliminary behavioral observations suggest that the influence of hormones on female-directed behavior may be more apparent in group-living monkeys than in individually caged individuals. This 1-year research project is aimed at investigating, through a combination of longitudinal and cross-sectional approaches, the influence of estradiol and progesterone on female responsiveness to infants during pregnancy in group-living rhesus and pigtail macaques. The hypotheses tested in this study are that pregnancy hormones enhance female motivation to approach and respond positively to infant stimuli and that individual differences in hormone concentrations during pregnancy may be related to quantitative, and perhaps qualitative, differences in female interactions with infants. The first experiment will investigate longitudinal changes in circulating estradiol/progesterone and infant handling in 10 pigtail macaque females. The second experiment will compare responsiveness to infants in 18 rhesus macaque females in five different hormonal conditions: (1) gonadally intact, nonpregnant; (2) gonadally intact, pregnant; (3) ovariectomized; (4) ovariectomized and treated with estradiol (5) ovariectomized and treated with estradiol and progesterone. Estradiol and progesterone will be administered in concentrations similar to those of late pregnancy. This research could provide the first experimental evidence that gonadal hormones enhance maternal responsiveness during pregnancy in macaques and important background information for future investigations of endocrine regulation of maternal responsiveness in primates and humans.